Abstract

Deposit-feeding gastropods provide integral links between benthic productivity and higher trophic levels in a variety of aquatic environments. At high population densities these snails can exert a significant grazing impact on available microphytobenthos (MPB). Here we provide the first report on the potential ingestion rates and feeding impact of the freshwater thiarid Melanoides tuberculata. This species consists of a variety of distinct morphs that have been introduced and become invasive in many areas in the New World. We used an in situ fluorometric approach to elucidate the feeding dynamics of three M. tuberculata populations from different habitat types. Daily ingestion rates, potential daily feeding impact and per capita resource availability were calculated based on snail density and MPB biomass. The highest maximum potential ingestion rate was estimated when snails occurred at the lowest population density, where per capita resource availability was highest. Feeding impact, in terms of potential daily consumption of MPB, varied depending on gut passage time and pigment consumption/digestion efficiency. Overall, the feeding dynamics of M. tuberculata were significantly different between populations. The variation in traits related to feeding dynamics, whether plastic or adaptive, contributes to the generalist nature of M. tuberculata. It is expected that these traits facilitate invasion success, as the species is able to inhabit and persist in a range of aquatic environments under different levels of resource availability.

INTRODUCTION

Benthic primary producers provide an essential autochthonous resource in shallow water ecosystems. This resource is ultimately linked to higher trophic levels through deposit-feeders, such as gastropods. Deposit-feeders are generally indiscriminate while feeding and thus often exhibit a large degree of trophic plasticity. The degree of trophic plasticity evident in most benthic aquatic gastropods is considered an important trait influencing the success of some species following their introduction into habitats outside their native range. The invasion success of apple snails (Ampullariidae), such as Pomacea canaliculata and P. maculata, the New Zealand mud snail Potamopyrgus antipodarum and the thiarid Tarebia granifera, has been attributed at least in part to the broad generalist diets of these species (Baker, Zimmanck & Baker, 2010; Miranda & Perissinotto, 2012; Bennett et al., 2015). Grazers at high densities have a substantial impact on available resources (Hillebrand, 2009), including top-down control on primary productivity and effects on community structure of benthic macrofauna as a result of competition (Kerans, Cada & Zickovich, 2010).

Melanoides tuberculata (Müller, 1774) is renowned as one of two globally invasive thiarid gastropods (Facon et al., 2003). The native range of M. tuberculata extends through East Africa, across the Middle East and to Southeast Asia (Brown, 1994; Facon et al., 2003). Within this range it is possible to find genetically and phenotypically distinct morphs (Samadi et al., 1999), which may have either African or Asian evolutionary origins due to human-mediated spread (Facon et al., 2003). Melanoides tuberculata typically occurs in a wide variety of perennial or temporary, freshwater or brackish habitats including rivers, streams, springs, wetlands, pans and coastal lakes (Brown, 1994; Appleton, 1996; de Kock & Wolmarans, 2009; Perissinotto et al., 2014). It is currently unknown whether the generalist habitat and dietary traits recorded for M. tuberculata are in fact due to intrinsic variations between genetically distinct morphs. Regardless, following establishment M. tuberculata attains high population densities (Work & Mills, 2013) and successfully displaces native gastropods, presumably through interspecific competition (Guimarães, de Souza & Soares, 2001).

Relatively little is known about the role M. tuberculata plays as a benthic consumer of primary production except that individuals feed by indiscriminately scraping the substrate for detritus, sedimentary organic matter and photosynthetic benthic microalgae (Madsen, 1992; Coat et al., 2009; Miranda & Perissinotto, 2012). Laboratory studies report that grazing by M. tuberculata influences the species richness and density of periphyton (Vasconcelos et al., 2013). However, the potential feeding impact of M. tuberculata on standing stocks of microphytobenthos (MPB) has not previously been investigated. The aim of this study was to estimate the feeding dynamics of M. tuberculata and to determine the impact of this species on available benthic resources. Feeding dynamics were assessed and compared for three distinct populations from environments that are representative of the variety of habitats in which the species occurs. It was predicted that feeding dynamics would differ between populations in different habitats in two ways. First, as M. tuberculata is a generalist deposit feeder, the average gut pigment content of individuals would be related to the available biomass of microalgae in each habitat. Second, the rate at which M. tuberculata consumed microalgae would vary with population density. Variation in feeding dynamics between different populations of M. tuberculata would be indicative of plasticity in these traits. This plasticity could be intrinsic or in response to environmental parameters, such as resource availability. The estimation of the potential feeding impact of M. tuberculata on microalgal resources should provide useful information relating to the invasion success of this species.

MATERIAL AND METHODS

Site description and gastropod collection

The iSimangaliso Wetland Park, a UNESCO World Heritage Site on the north coast of KwaZulu-Natal Province, South Africa, protects a wide range of coastal habitats including three major Ramsar Wetlands of International Importance (Fig. 1).

Figure 1.

Map of the iSimangaliso Wetland Park and morphological variation between Melanoides tuberculata collected for feeding experiments from the St Lucia Estuary Mouth (A), the Mpophomeni Stream (B) and Lake Nhlange (C).

Figure 1.

Map of the iSimangaliso Wetland Park and morphological variation between Melanoides tuberculata collected for feeding experiments from the St Lucia Estuary Mouth (A), the Mpophomeni Stream (B) and Lake Nhlange (C).

For this study, in situ experiments were carried out with Melanoides tuberculata collected from three different habitats: (1) the mangrove forest on the north shore of the St Lucia Estuary Mouth; (2) the muddy bed of the Mpophomeni Stream in the False Bay region of Lake St Lucia; (3) the sandy shores of Lake Nhlange in the Kosi Bay lake system. These habitats are representative of the range of environments in which M. tuberculata typically occurs in terms of temperature, salinity and substrate type. Although M. tuberculata is indigenous to South Africa (Brown, 1994), preliminary conchological analyses using the categorical scoring system of Facon et al. (2003) indicate that there are currently several morphologically distinct populations. In this study (Fig. 1: inset), M. tuberculata from the St Lucia Estuary Mouth (A) as well as the Mpophomeni Stream (B) have been identified as the characteristic tuberculate form, which is considered to be indigenous (see Appleton, 1996). However, those from Lake Nhlange (C) display a smooth sculpture and have a distinct columellar band, resembling the Asian genotype/morph illustrated by Genner et al. (2004). This is the subject of an ongoing study in South Africa (Appleton & Miranda, 2015).

The experiments with M. tuberculata from the St Lucia Estuary Mouth and the Mpophomeni Stream were carried out on 21 and 23 February 2014, respectively. The experiments with M. tuberculata from Lake Nhlange were carried out on 1 February 2015. Physicochemical parameters were measured upon arrival at each site using a YSI 6600-V2 multiprobe (Table 1). Approximately 10 individual snails were collected and then processed to obtain an estimate of gut pigment content at that time.

Table 1.

Average (± SE) values of physicochemical variables, population parameters and gut pigment content for Melanoides tuberculata at St Lucia Estuary Mouth, Mpophomeni Stream and Lake Nhlange.

Site Date and time Salinity Temperature (°C) Gut pigment content* (μg pigm. ind−1Average SH (mm) Density (ind.m−2
St Lucia Estuary Mouth 21 February 2014
10:00 
7.62±0.04 25.90±0.25 83.36±13.15 13.37±0.58 288±15 
Mpophomeni Stream 22 February 2014
16:00 
8.83±0.02 24.64±0.07 338.86±39.18 26.17±0.92 67±7 
Lake Nhlange 31 January 2015
14:00 
2.92±0.03 32.40 ±0.11 255.07±69.94 13.22±0.56 104±34 
Site Date and time Salinity Temperature (°C) Gut pigment content* (μg pigm. ind−1Average SH (mm) Density (ind.m−2
St Lucia Estuary Mouth 21 February 2014
10:00 
7.62±0.04 25.90±0.25 83.36±13.15 13.37±0.58 288±15 
Mpophomeni Stream 22 February 2014
16:00 
8.83±0.02 24.64±0.07 338.86±39.18 26.17±0.92 67±7 
Lake Nhlange 31 January 2015
14:00 
2.92±0.03 32.40 ±0.11 255.07±69.94 13.22±0.56 104±34 

*Gut pigment content estimated from the 10 individuals collected immediately upon arrival at each site.

Abbreviation: SH, shell height.

Gastropod grazing experiments

Gut contents

To determine the dominant constituents in the diet of M. tuberculata, the gut contents of 15 randomly selected individuals from each site were examined under a dissecting microscope. Gut contents were classified by gross morphology (microalgae, filamentous algae, detritus, sediment) and the percentage of snails that contained these items in their guts was calculated.

To determine natural variations in the pigment in the gut (a proxy for ingested photosynthetic material), individuals were collected at 3-h intervals over a period of 24 h. Five individuals were collected after each time interval and the shell height (SH) of each snail was measured with Vernier callipers. The gastropods were dissected and each whole gut was placed in 8 ml of 90% acetone and then stored in the dark at 4 °C for 48 h to extract chlorophyll a. The total pigments (chlorophyll a and phaeopigments) were subsequently measured using the nonacidification method with a 10-AU Turner Designs fluorometer (Welschmeyer, 1994) and expressed as chlorophyll a equivalents (μg pigm.ind−1).

Gut chlorophyll content was expected to vary with shell size. Therefore, to determine whether gut chlorophyll content varied over the course of the day at each site, time period (morning, afternoon, evening, night) was used as a categorical predictor variable in a general linear model (GLM), which incorporated SH as a continuous covariate. The slope of the linear relationship between gut chlorophyll content and shell size was not consistent between sites. Therefore, to compare the overall difference in gut chlorophyll content between sites the measurements were size-standardized across all three populations to allow for suitable comparisons using the mean SH as follows: 

Gutpigmentcontent(μgpigm.ind1)=chlorophyll a equivalents×(meanSH/individualSH)

ANOVA with Tukey's HSD post hoc test was used to compare gut pigment content between populations. Size-standardized gut pigment values were used to calculate all subsequent parameters in order to make these values directly comparable between populations.

The feeding rate of M. tuberculata was estimated using the in situ gut-fluorescence technique developed by Mackas & Bohrer (1976), which has since been adapted for gastropods (Miranda, Perissinotto & Appleton, 2011; Díaz, Kraufvelin & Erlandsson, 2012). The daily ingestion rate (I, mg pigm.ind−1 d−1) was estimated as follows: 

I=kG/(1b)

Where k is the gut evacuation rate (h−1), G is the integrated average size-standardized gut pigment concentration (adjusted to mg pigm.ind−1) over 24 h and b is an index of pigment consumption/digestion within the gut (Wang & Conover, 1986; see below). The calculated values for I were compared between populations using Pearson's chi square test.

Gut evacuation rate (k)

The gut evacuation rate was measured with freshly collected M. tuberculata. Individuals were isolated in 100 ml plastic vials containing water collected in situ and filtered through a Whatman GF/F (0.7 μm) and a Millipore filter (0.2 µm), to ensure removal of all particulate material. To promote continuous gut evacuation, nonfluorescent cornstarch was added as a source of food to the filtered water (Carrasco & Perissinotto, 2010). The gut pigment content was measured (see above) from five gastropods at the beginning of the experiment. Five individuals were subsequently processed every 10 min for the first hour and every 30 min for the following 2 h (total duration 3 h). The gut evacuation rate was estimated from the linear slope of the change in gut pigment over time (Pakhomov & Perissinotto, 1996). To determine maximum potential ingestion rates (Imax), the maximum gut evacuation rate (kmax) was estimated from the rate of pigment decline over the first 30 min of the experiment.

Pigment consumption/digestion (b)

The efficiency of M. tuberculata in consuming/digesting photosynthetic pigments in the gut was determined using the two-compartment pigment-budget approach (Mayzaud & Razouls, 1992). Twenty individuals were isolated in 100 ml of the filtered water and cornstarch mixture described above for a period of 24 h. The water-cornstarch solution was replaced after 12 h to avoid a buildup of waste products. After the 24-h period, the gut pigment content was measured from ten individuals. A 20-ml suspension of microalgae was added to each of ten 2-l buckets containing 300 ml of pure filtered water (no cornstarch) collected in situ. After the microalgae had settled, the remaining 10 snails were each placed in a bucket and allowed to feed for a period of 1 h. A control was set up in the same manner but without the addition of gastropods. After the incubation period of 1 h, the snails were processed as described above. The water from each replicate was filtered (Whatman GF/F 0.7 µm) and the filter was placed in 8 ml of 90% acetone for extraction as described. No faecal pellets were produced during the incubation period. The differences in pigment concentration between the water and snail compartments were therefore attributed to consumption or digestion within the gut.

Gastropod abundance, resource availability and impact on MPB

Gastropod abundance was measured quantitatively using a corer with an internal diameter of 42 mm at the St Lucia Estuary Mouth and the Mpophomeni Stream. The corer was pushed into the sediment and all individuals within the area were counted. This method could not be employed at Lake Nhlange where M. tuberculata occurs in deeper water. Instead, a net of 0.3 m diameter was swept across 5 m of the substrate where M. tuberculata occurred. Abundance sampling was replicated in triplicate at each site to estimate population density (ind.m−2).

The potential feeding impact (mg pigm.m−2 d−1) was calculated as the product of the daily ingestion rate and the density of gastropods for each population. The average available MPB standing stock (mg pigm.m−2) was estimated from triplicate cores (20 mm internal diameter) placed in 30 ml of 90% acetone at 4 °C for 48 h. After chlorophyll a extraction, the pigment concentrations were measured fluorometrically as described above. The feeding impact of gastropods was then expressed as a percentage of the average available MPB standing stock. The biomass of MPB available per individual snail (mg pigm.m−2 ind−1) was estimated as an indication of resource availability at each site. The estimates for feeding impact were compared between populations using Pearson's chi square tests.

RESULTS

Gut pigment content

The gut contents of Melanoides tuberculata were dominated by microalgae and detritus for snails from all three populations (Table 2). Filamentous algae only occurred in the guts of snails from the Mpophomeni Stream, although their percentage was relatively low.

Table 2.

Percentage occurrence of gut content items for Melanoides tuberculata from the St Lucia Estuary Mouth, Mpophomeni Stream and Lake Nhlange.

Site Microalgae Filamentous algae Detritus Sediment 
St Lucia Mouth 73.3 – 80 6.7 
Mpophomeni Stream 72.2 16.7 61.1 33.3 
Lake Nhlange 64.3 – 78.6 42.9 
Site Microalgae Filamentous algae Detritus Sediment 
St Lucia Mouth 73.3 – 80 6.7 
Mpophomeni Stream 72.2 16.7 61.1 33.3 
Lake Nhlange 64.3 – 78.6 42.9 

The average gut-pigment content of individuals collected immediately upon arrival differed between sites. The average size of individuals was relatively similar at the St Lucia Estuary Mouth and Lake Nhlange, while those at the Mpophomeni Stream were double this size (Table 1). Although SH was a significant covariable of gut chlorophyll content at St Lucia Estuary Mouth (F1,37 = 4.249, P = 0.047), Mpophomeni Stream (F1,38 = 5.515, P = 0.025) and Lake Nhlange (F3,36 = 6.2716, P = 0.017), these parameters were in fact poorly correlated at each site (r2 = 0.162; r2 = 0.145 and r2 = 0.228 respectively).

Gut chlorophyll content did not differ significantly during the course of the day for M. tuberculata at Lake Nhlange (GLM: F3,36 = 0.982, P = 0.413), Mpophomeni Stream (GLM: F3,38 = 451, P = 0.718) or St Lucia Estuary Mouth (GLM: F3,37 = 1.362, P = 0.271) (Fig. 2). However, the average size-standardized gut chlorophyll measured over 24 h (G) between snails was higher at the Mpophomeni Stream (ANOVA: F2,126 = 27.745, P = 0.001) in comparison with those from the St Lucia Estuary Mouth and Lake Nhlange (Table 3).

Table 3.

Integrated size-standardized average gut pigment content measured over 24 h, maximum gut evacuation rate (with corresponding gut passage time) and pigment consumption/digestion efficiency calculated for Melanoides tuberculata from different localities.

Site Gut pigment content (G) (mg pigm. ind−1Gut evacuation rate (kmax) (h−1Gut passage time (h) Pigment consumption/digestion efficiency (b) (%) 
St Lucia Estuary Mouth 1.38 0.48 2.08 11.1 
Mpophomeni Stream 3.93 0.42 2.38 87.7 
Lake Nhlange 1.40 0.54 1.85 84.1 
Site Gut pigment content (G) (mg pigm. ind−1Gut evacuation rate (kmax) (h−1Gut passage time (h) Pigment consumption/digestion efficiency (b) (%) 
St Lucia Estuary Mouth 1.38 0.48 2.08 11.1 
Mpophomeni Stream 3.93 0.42 2.38 87.7 
Lake Nhlange 1.40 0.54 1.85 84.1 
Figure 2.

Diel variation in gut pigment content for Melanoides tuberculata at the St Lucia Estuary Mouth, the Mpophomeni Stream and Lake Nhlange.

Figure 2.

Diel variation in gut pigment content for Melanoides tuberculata at the St Lucia Estuary Mouth, the Mpophomeni Stream and Lake Nhlange.

Ingestion rates, feeding impact and resource availability

A negative exponential model provided the best fit for the rate of pigment decline over a 3-h period for all experiments (Fig. 3). The maximum gut evacuation rate (kmax) varied between sites and was highest for snails at Lake Nhlange and lowest at the Mpophomeni Stream (Table 3). Both the average gut evacuation rate (k) (Fig. 3) and the calculated average ingestion rate (I) were similar (χ2 = 1.455, df = 2, P = 0.483) between populations (Table 4). This was the result of the variation in the consumption/digestion efficiency (b) and the size-standardized gut chlorophyll content (G) between snails from different sites (Table 3). As a product of density and ingestion rate, the average feeding impact did differ significantly (χ2 = 36.602, df = 2, P < 0.0001) between sites (Table 4). The maximum potential feeding impact (estimated from Imax) was also significantly different between sites (χ2 = 471.077, df = 2, P < 0.0001) (Table 4). Biomass of MPB was similar at the St Lucia Estuary Mouth and the Mpophomeni Stream (Table 4). As M. tuberculata occurred at the lowest density at the Mpophomeni Stream where MPB biomass was relatively high, the per capita resource availability (mg pigm.m−2 ind−1) was greatest at this site (Table 4). The average potential daily consumption of MPB differed significantly between sites (χ2 = 39.942, df = 2, P < 0.0001) and was highest at Lake Nhlange (Table 4).

Table 4.

Calculated average and maximum values for ingestion rate and feeding impact for Melanoides tuberculata at St Lucia Estuary Mouth, the Mpophomeni Stream and Lake Nhlange. Average (± SE) available microphytobenthos (MPB) biomass, potential daily MPB consumption and per-capita availability of MPB at each site are also given.

Site Ingestion rate (mg pigm. ind−1 d−1Feeding impact (mg pigm. m−2 d−1MPB biomass (mg pigm. m−2Daily MPB consumption (%) Per-capita available MPB (mg pigm. m−2ind−1
St Lucia Estuary Mouth 0.17 48.30 436.82±213.72 11.06 1.52 
0.67 193.19    
Mpophomeni Stream 1.91 128.0 415.14±76.46 30.83 6.20 
13.37 896.02    
Lake Nhlange 1.05 109.86 173.25±35.08 63.41 1.67 
4.75 494.39    
Site Ingestion rate (mg pigm. ind−1 d−1Feeding impact (mg pigm. m−2 d−1MPB biomass (mg pigm. m−2Daily MPB consumption (%) Per-capita available MPB (mg pigm. m−2ind−1
St Lucia Estuary Mouth 0.17 48.30 436.82±213.72 11.06 1.52 
0.67 193.19    
Mpophomeni Stream 1.91 128.0 415.14±76.46 30.83 6.20 
13.37 896.02    
Lake Nhlange 1.05 109.86 173.25±35.08 63.41 1.67 
4.75 494.39    
Figure 3.

Decrease in gut pigment content over time for Melanoides tuberculata from St Lucia Estuary Mouth (t0 = 12:00) (A), Mpophomeni Stream (t0 = 14:00) (B) and Lake Nhlange (t0 = 16:00) (C). Average gut evacuation rate (k) calculated as the linear slope of log-transformed gut pigment content over the entire 180 min.

Figure 3.

Decrease in gut pigment content over time for Melanoides tuberculata from St Lucia Estuary Mouth (t0 = 12:00) (A), Mpophomeni Stream (t0 = 14:00) (B) and Lake Nhlange (t0 = 16:00) (C). Average gut evacuation rate (k) calculated as the linear slope of log-transformed gut pigment content over the entire 180 min.

DISCUSSION

Melanoides tuberculata has been reported to consume periphytic biofilms (Vasconcelos et al., 2013) as well as detritus and microalgae (Coat et al., 2009). However, this is the first report on the rate at which M. tuberculata is able to consume available MPB. Remarkably, this species was able to consume over 60% of the total available microalgal biomass in one of the three different systems (Lake Nhlange) investigated in this study. There was, however, a large degree of variability in the feeding dynamics of M. tuberculata from the different habitats.

Differences in feeding dynamics between populations

Food availability and quality of resources

The consumption of benthic microalgae by M. tuberculata was not directly related to the biomass of available MPB. However, when considering the variation in per-capita available MPB between sites there is a clear trend in relation to gut pigment content (G). Population density has the potential to influence ingestion rates of deposit-feeding gastropods, as the result of interference competition through space limitation and an increase in the frequency of interactions between individuals (Blanchard et al., 2000). These effects were, however, not recorded for snails at natural densities (Barnes, 2001). Lower gut pigment content measured at high population density (when per-capita available MPB was low) at the St Lucia Estuary Mouth was related to a lower ingestion rate and subsequently a lower potential impact on available stocks. However, a high pigment consumption/digestion efficiency coupled with a fast gut passage time resulted in a high calculated ingestion rate at Lake Nhlange. Under these conditions when the total available biomass of MPB was lower, which also resulted in a low per-capita availability of MPB, the potential feeding impact of snails was in fact very high.

As M. tuberculata is a generalist deposit feeder, the lower ingestion rates measured at higher population densities may be the result of these individuals consuming a lower proportion of photosynthetic material. Detritus was indeed recorded in the gut contents from a large percentage of the snails examined from all three habitats. The constituents of particulate detritus can differ significantly among habitats (Yee & Juliano, 2006). However, detritus is a nutritionally poor food source for gastropods in comparison with benthic microalgae (Levinton, Bianchi & Stewart, 1984). In the mangrove habitat, M. tuberculata may be fulfilling their energetic requirements by feeding predominantly on sediment enriched with organic detritus and microbes, which are common alternatives to photosynthetic microalgae for deposit feeding gastropods (Sheldon & Walker, 1997).

The nutritional quality of food, as well as the physical structure and complexity of food particles, have been related to variations in gut passage time and the efficiency with which individuals consume/digest pigments within the gut (Taghon & Jumars, 1984). Gut passage time is generally positively correlated with this consumption/digestion efficiency (Hawkins, Navarro & Iglesias, 1990). Therefore, a long gut passage time could be indicative of either poor food quality or high structural complexity of the food consumed. Similarly, different algal groups are digested in the gut at different rates and efficiencies by gastropods (Brendelberger, 1997). In the case of M. tuberculata from the Mpophomeni Stream, filamentous Cladophora was recorded in the gut contents from some individuals even though they were collected from outside the patches of this alga. Although microalgae were most commonly found in the guts, the incorporation of Cladophora potentially leads to higher consumption/digestion efficiency in these snails. Therefore, although the gut passage time was longer for M. tuberculata from the Mpophomeni Stream, their high efficiency contributed to a faster ingestion rate and thus to a significantly larger potential feeding impact on available MPB.

Environmental variation

The variation in feeding dynamics observed for M. tuberculata from different habitats is potentially driven by a range of environmental conditions. Temperature is positively correlated with ingestion rates in gastropods (Foster & Hodgson, 1998; Sanford, 2000). As temperature drives metabolic processes, it would be expected also to influence gut passage time and consumption/digestion efficiency. The fastest gut passage time (estimated from kmax) and highest consumption/digestion efficiency were recorded from Lake Nhlange where environmental temperature was indeed the highest. The calculated ingestion rate was not, however, directly correlated with the temperature differences between habitats. Ingestion rates were in fact highest under the highest environmental salinity, which was recorded at the Mpophomeni Stream. The feeding activity of benthic invertebrates generally varies within optimal ranges of salinity (Irlandi, Maciá & Serafy, 1997; Pascal et al., 2008). Although M. tuberculata is considered to be a predominantly freshwater species, it has been repeatedly reported from estuarine habitats and the species has a broad salinity tolerance (da Silva & Barros, 2015). The relationship between salinity and ingestion rate may therefore determine the extent to which M. tuberculata is able to affect benthic assemblages following introduction.

Substrate type and, in particular, grain size determine the surface area available for colonization and thus influence the biomass of MPB in aquatic habitats (Cahoon, Nearhoof & Tilton, 1999). Sediment particles are also indirectly ingested by grazers (Levinton et al., 1984) with implications for digestion/consumption efficiency within the gut (Broekhuizen, Parkyn & Miller, 2001). A third of the individuals examined from the Mpophomeni Stream contained sediment particles within their guts. However, for M. tuberculata, sediment was most frequently found in the gut contents of individuals collected from the sandy habitat at Lake Nhlange. A fast gut passage time for these snails may therefore facilitate fast removal of indigestible particles.

Evolutionary significance of variation in feeding dynamics

The large degree of variation in feeding dynamics recorded for M. tuberculata suggests that traits related to feeding may be plastic and therefore that they may differ between environments. However, this can only be concluded following a specific experimental investigation, which was beyond the scope of this study. Phenotypic plasticity arises when individuals of a similar genotype express different traits depending on environmental conditions (Scheiner, 1993). This is often difficult to assess in natural populations, except in the situation of clonal species in which populations tend to be dominated by closely related individuals (Jackson, 1986). In the case of M. tuberculata, which reproduces through ovoviviparous parthenogenesis, populations may indeed be genetically similar (Myers, Meyer & Resh, 2000). The existence of genetically distinct but highly plastic morphs within this species (Samadi et al., 1999) complicates this scenario. Although certain traits may be expected to be plastic, there may be genetic differences between morphs that have experienced different selective pressures.

Differences in physiology and other traits that would influence competition have not been directly compared between African and Asian morphs of M. tuberculata. However, at the time of their study, Genner et al. (2004) reported that morphs of Asiatic origin presently occur in areas of Lake Malawi that were previously occupied by native African morphs. This suggests that Asian morphs have displaced their African counterparts. Within the iSimangaliso Wetland Park, M. tuberculata has been displaced by the alien invasive Tarebia granifera (Miranda & Perissinotto, 2014), with the exception of those at Lake Nhlange. Individuals of M. tuberculata from Lake Nhlange also behave differently from those from St Lucia Estuary Mouth (Raw, Miranda & Perissinotto, 2015). A shared Asian evolutionary origin between T. granifera and certain M. tuberculata morphs may explain these variations.

Globally, M. tuberculata has been introduced to many subtropical and tropical locations such as the southeastern United States (Karatayev et al., 2009), the Caribbean (Pointier, David & Jarne, 2011) as well as areas of South America in Brazil, Argentina and Paraguay (De Marco, 1999; Peso, Pérez & Vogler, 2011). The invasion success of M. tuberculata has largely been attributed to advantageous functional traits including wide physiological tolerance of desiccation (Facon et al., 2004), temperature (Mitchell & Brandt, 2005) and salinity (Weir & Salice, 2012). If traits relating to feeding are indeed plastic, this would be advantageous to the success of M. tuberculata as an invasive species. The variation in feeding dynamics suggests that these snails utilize resources at different rates depending on their availability. However, individuals exhibit a degree of trophic plasticity as indicated by the various gut content items. As such, M. tuberculata has the ability to occupy a relatively broad trophic niche. Therefore, the invasion success of M. tuberculata may in part be attributed in the ability of this species to exploit a range of resources at variable rates.

Comparisons with other globally invasive species

High densities of invasive gastropod species such as Potampopyrgus antipodarum, T. granifera and M. tuberculata are considered competitive threats to native species when food resources are limited (Rader, Belk & Keleher, 2003; Appleton, Forbes & Demetriades, 2009; Moore et al., 2012). Our results show that the daily consumption of available MPB by M. tuberculata is higher than that reported for the closely related T. granifera by Miranda et al. (2011) (63.4 and 35%, respectively). In contrast, P. antipodarum is potentially able to consume up to 75% of the daily available gross primary production (Hall, Tank & Dybdahl, 2003).

Although ingestion rates are typically highest at low population densities, when per-capita resource availability is high, grazing impact on available algal stocks by P. antipodarum has also been positively correlated with primary productivity within areas of their introduced range (Riley, Dybdahl & Hall, 2005). Similarly, for T. granifera, grazing impacts were positively correlated with available MPB biomass (Miranda et al., 2011). In contrast, a high grazing impact was recorded for M. tuberculata at Lake Nhlange, which is oligotrophic (Begg, 1980), as the result of a fast ingestion rate and relatively high population density. It may be that snails from Lake Nhlange have higher ingestion rates in order to maximize consumption of limited food resources. At Lake Nhlange in particular, Miranda et al. (2011) reported that T. granifera only consumed up to 27.4% of the available MPB biomass (37.3±25.7 mg pigm.m−2) while present at a density of 282±66 ind.m−2. In comparison, at a density of 104±34 ind.m−2, M. tuberculata was estimated to consume 63.4% of available MPB (173.25±35.1 mg pigm.m−2). This possibly suggests that under conditions of low per-capita resource availability T. granifera has a higher carrying capacity within this oligotrophic habitat. This may be contributing to the disappearance of M. tuberculata from other water bodies within this biogeographic region following the introduction of T. granifera (Miranda & Perissinotto, 2014).

CONCLUSION

The success of M. tuberculata following introduction and establishment is expected to be the result of a combination of traits. In this study, we found that the feeding dynamics of M. tuberculata was variable between populations from different habitats. This may be an important factor contributing towards the invasion success of this species, as it is able to utilize available resources at different rates. It is evident that these results should be complemented with a stable isotope analysis to determine both immediate and long term contributions of various food resources to the diet of M. tuberculata in different aquatic habitats. However, as MPB is generally the most important basal resource in aquatic environments, the in situ gut fluorescence method provides a good estimate of the potential impact of grazing snails.

ACKNOWLEDGEMENTS

Gastropods were collected on an integrated Environmental and Fisheries Research Permit (RES2013/13) issued by DAFF (South African Department of Agriculture, Forestry and Fisheries) under Section 83 of the Marine Living Resources Act. All necessary permits were obtained from the iSimangaliso Wetland Park Authority for the described experiments. The study was conducted with the clearance of the Research Ethics Committee of the Nelson Mandela Metropolitan University (ref: A14-SCI-ZOO-009). We thank the iSimangaliso Wetland Park Authority and Ezemvelo KZN Wildlife for supporting this research. Additional thanks go to Nicola Carrasco, Holly Nel and Salome Jones for their various recommendations as well as to Matthew Bird and Bradley Ah Yui who assisted with field experiments. We are also grateful to reviewers Romi Burks, Martin Genner and Alex Yeung for detailed, constructive input that strengthened the paper. Funding was provided by the National Research Foundation (NRF) of South Africa and the Nelson Mandela Metropolitan University. This work is based on the research supported by the South African Research Initiative of the Department of Science and Technology and the NRF.

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